The specific aim of this project is to measure calcium flux in single cells by phase-sensitive flow cytometry using calcium indicator dyes, such as, Indo-1 and Calcium Green. At present, calcium flux measurements are made using both single and dual-laser excitation coupled with ratiometric fluorescence detection methods employing optical bandpass filters and analog ratioing electronics. We will explore the utilization of differences in lifetimes between bound and unbound calcium probes to resolve fluorescence emission signals and thus eliminate the need for ratiometric measurements to get the concentration of a cellular constituent.